feat: add ETag cache optimization, code-split WebUI, and refactor XML tool scanner

- Chat history: early 304 via Revision()/DetailRevision() to avoid full snapshot reads
- WebUI: lazy-load tab containers with Suspense fallback
- Toolstream: split tool_sieve_xml.go into tags.go and scan.go
- CI: trigger on main branch, guard cross-build to dev/main pushes only
- Docs: add DEVELOPER.md developer quick reference

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/quality-gates.yml

@@ -5,6 +5,7 @@ on:
   push:
     branches:
       - dev
+      - main
 
 permissions:
   contents: read
@@ -114,6 +115,7 @@ jobs:
 
   cross-build:
     name: Release Target Cross-Build
+    if: ${{ github.event_name == 'push' && (github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main') }}
     runs-on: ubuntu-latest
     steps:
       - name: Checkout